1. Field of the Invention
This invention relates to a polarizing beam splitter apparatus commonly usable as a polarizer and an analyzer and a light valve image projection system using the same in which an optical image formed on a reflection light valve is irradiated by an illuminating light to be projected on a projection screen through a projection lens.
2. Description of the Prior Art
As a method to obtain a large projection screen image, projection display system using a liquid crystal panel as the light valve is well-known conventionally. In this system, an optical image corresponding to an image signal is formed on a liquid crystal panel and the optical image thus formed thereon is irradiated by an illuminating light and enlarged and projected on a projection screen by a projection lens. A projection display system in which a reflection liquid crystal panel is used for the light valve is disclosed in, for example, Japanese Pat. Appln. of Laid-Open No. 61-13885 and U.S. Pat. No. 4,464,018. The optical system of this projection display system consists of a light source for irradiating an approximately parallel light, a polarizing beam splitter to be commonly used as a polarizer and an analyzer, a reflection liquid crystal panel and a projection lens. A light outputted from the light source is separated by the polarizing beam splitter into an S polarized component to be reflected and a P polarized component to be passed straight. The S polarized light component thus obtained is incident to the liquid crystal panel and the reflection light therefrom is sent to the polarizing beam splitter again. The P polarized component contained in the reflection light passes straight through the polarizing beam splitter to enter into the projection lens. The reflection liquid crystal panel uses the birefringence of a liquid crystal and has a reflection electrode for reflecting light. The reflection liquid crystal panel is not substantially birefringent when a voltage is not applied to the liquid crystal layer, but when a voltage is applied thereto, birefringent characteristics are realized. If a linearly polarized light having a pre-determined polarization direction is directed into the liquid crystal panel, the polarization state of light reflected therefrom can be controlled by the voltage that is applied thereto. As such an optical image can be formed on the liquid crystal panel as the result of the change in birefringence and the optical image thus formed can be enlarged and projected on a projection screen by the projection lens.
In the reflection liquid crystal panel, a switching element can be arranged under the pixel electrode, so that a high density arrangement is made possible by reducing the pixel pitch without the need to make the switching element small, thus being advantageously capable of obtaining a projected image of a higher resolution on a projection screen than in the case of using a transmission liquid crystal panel.
With the light valve image projection system as such, however, if the angle between the light incident to the polarizing beam splitter and the incident optical axis is increased, a problem may arise in that the contrast of a projected image is reduced. Therefore, a xenon lamp, which has a small light radiator and is high in luminance, has been used in many cases. In this case, however, it has been pointed out that the xenon lamp is low in efficiency and short in service life. From the viewpoint of the efficiency and service life of a lamp to be used, the use of a metal halide lamp can be considered, but such is a larger light radiator as compared with the xenon lamp, so that the angle between the incident light to the polarizing beam splitter and the incident optical axis is increased, the result being that the brightness and the contrast of a projected image are difficult to coexist.